Conventionally, a ship is known having an inboard motor (inboard engine, outboard drive) in which a pair of left and right engines are arranged inside a hull and power is transmitted to a pair of left and right outdrive devices arranged outside the hull. The outdrive devices are propulsion devices rotating screw propellers so as to propel the hull, and are rudder devices rotated concerning a traveling direction of the hull so as to make the hull turn.
Such outdrive devices are rotated with hydraulic steering actuators provided in the outdrive devices (for example, see the Patent Literature 1). Then, a rotation angle of each of the outdrive devices, that is, a steering angle is grasped based on detection results of an angle detection sensor and the like provided in a linkage mechanism constituting the outdrive device.
The ship has an operation means setting a traveling direction of the ship. The ship is controlled with a control device so as to travel to the direction set with the operation means.
However, when the operation means is operated so as to make the ship sail obliquely, a pressure center of the hull is not in agreement with a centroid of the hull, whereby a lifting power is generated at a position of the hull shifted from the centroid. Accordingly, unintended rotation of the hull (yawing, turning) is caused. Since the influence is different concerning type, size and apparatus mounting position of the hull, a suitable correction value for canceling the unintended rotation of the hull cannot be determined uniquely. Accordingly, it is necessary to determine the suitable correction value for canceling the unintended rotation of the hull about each ship.
The ship described in the Patent Literature 1 is constructed so as to be moved laterally with propulsion power of a pair of outdrive devices by forward rotation of one of the outdrive devices and reverse rotation of the other outdrive device.
In such a ship, for making the ship move laterally without turning, it is necessary to make a resultant of the propulsion power of the left outdrive device and the propulsion power of the right outdrive device (hereinafter, referred to as “total propulsion power”) act on the centroid of the hull. For making the total propulsion power act on the centroid of the hull, it is necessary to rotate the left outdrive device and the right outdrive device respectively so as to make an intersection of the direction of the propulsion power of the left outdrive device and the direction of the propulsion power of the right outdrive device in agreement with the centroid of the hull. When the intersection of the direction of the propulsion power of the left outdrive device and the direction of the propulsion power of the right outdrive device is not in agreement with the centroid of the hull, the total propulsion power does not act on the centroid of the hull, whereby the ship is not moved laterally and is turned.
In such a ship, for making the ship move laterally without turning, it is necessary to make the total propulsion power act on a direction to which the lateral movement of the ship is required. For making the total propulsion power act on the direction to which the lateral movement of the ship is required, it is necessary to make the propulsion power of the left outdrive device equal to the direction of the propulsion power of the right outdrive device. When the propulsion power of the left outdrive device is not equal to the direction of the propulsion power of the right outdrive device, the total propulsion power does not act on the direction to which the lateral movement of the ship is required, whereby the ship is not moved laterally and is turned.
Herein, since the centroid of the hull is different in each ship, the rotation angles of the outdrive devices at the time at which the intersection of the direction of the propulsion power of the left outdrive device and the direction of the propulsion power of the right outdrive device is in agreement with the centroid of the hull (hereinafter, referred to as “reference steering angle”) must be set corresponding to each ship. In the outdrive devices, the propulsion power generated by forward rotation is different from that generated by reverse rotation even if the rotation speed is common, whereby a ratio of the rotation speed of the left outdrive device and the rotation speed of the right outdrive device at the time at which the propulsion power of the left outdrive device is equal to the direction of the propulsion power of the right outdrive device (hereinafter, referred to as “reference propulsion power ratio”) must be set corresponding to each ship. Furthermore, since the reference steering angle and the reference propulsion power ratio are influenced of the shape of the hull and the weight of the ship intricately, the reference steering angle and the reference propulsion power ratio must be set by actual sailing of the ship, whereby an art is required for controlling the ship so as to perform the lateral movement easily.    Patent Literature 1: the Japanese Patent Laid Open Gazette 2005-114160